The present invention relates to a thermal toner fixing device for a xerographic apparatus.
In most xerographic apparatus, the surface of an electrostatically charged photosensitive drum is exposed to image carrying light to form a latent image thereon, toner is adhered to the latent image to develop it, and the toner image is transferred to a sheet of paper and fixed thereon.
The toner fixing operation in this process is commonly conducted using a thermal toner fixing device like the one shown in FIG. 3. This device has a heat roller 2 rotatably supported on a shaft 1 and heated by an appropriate heat source to a temperature of around 180.degree. C., and a silicon rubber pressure roller 5 supported on a shaft 4. The surface 3 of the heat roller 2 and the surface 6 of the pressure roller 5 are forced against each other at a prescribed pressure. When a sheet of paper 7 bearing a toner image is inserted between the rollers 2 and 5 and the rollers are independently rotated for passing the sheet therebetween, the toner is melted and fixed to the paper by the heat of the heat roller 2.
When a sheet of paper 7 that is narrower than the width (lateral length) of the heat roller 2 is passed between the rollers, the pressure roller 5 absorbs more heat at the portions where the paper is not present than at the portions where it is. The temperature at the surface of the pressure roller 5 therefore rises to around 180.degree. C. at portions where it is in direct contact with the heat roller 2, as compared with only around 50.degree. C. at portions where it indirectly contacts the heat roller 2 through the paper.
The diameter of the pressure roller 5 at the portions on opposite sides of the paper 7 therefore increases considerably owing to thermal expansion. As a result, the heat roller 2 is pushed upward, with the non-uniform heating producing an inclined step 8 (exaggerated in the figure for clarity) in the pressure roller 5 on either side of the portion in contact with the paper 7 and a gap 9 between the surface 3 of the heat roller 2 and the surface 6 of the pressure roller 5 at the portion where the paper is present. This is apt to give rise to slippage between the paper 7 and the rotating heat and pressure rollers so that the paper 7 is liable to be improperly conveyed and may jam.